Optical coherence tomography is a technique by which a partially transparent object, such as an eye, may be scanned in three dimensions using interferometric techniques to build up a complete three dimensional volume image of the eye. In time domain OCT, a sample beam from an interferometer defines a coherence gate in the region where the optical path difference is less than the coherence length. The sample beam is scanned over the eye, typically in a raster fashion, to obtain reflectance data at points in an X-Y plane located at a depth corresponding to the coherence gate. By moving the coherence gate in the Z direction, data from different X-Y planes can be obtained. In spectral OCT, a complete line of data in the Z direction can be obtained at each point in the X-Y plane. A typical time domain OCT apparatus is described in U.S. Pat. No. 6,769,769, the contents of which are herein incorporate by reference. U.S. Pat. No. 6,377,349, the contents of which are herein incorporated by reference, describes a method employing spectral OCT.
A number of scanning schemes are known as described in U.S. Pat. No. 5,321,501, the contents of which are also herein incorporated by reference. The eye can be scanned transversely (en face) in a raster fashion, with the coherence gate moved in the Z direction after each complete en face scan, or alternatively it can be scanned in the X-Z or Y-Z plane, to obtain a series of slices (B-scans) that are then assembled to produce a complete three dimensional image.
In order to build up a complete three dimensional volume image of the eye, it is important that the eye remain stationary. Absolute stability of the patient's eye is required. Normally the stoccatic movements of a patient's eye are not of great importance because the patient can be quickly trained to fixate on a target while the scan is performed. However when visible laser light is used to perform the scan, a patient's natural reaction is to follow the motion of the laser after it passes through their fovea (center of focus). One option would be to freeze the patient's eye, but that option is neither desirable nor feasible for a simple OCT scan.